Resource processing time

While talking to people about visual metrics and the reasons it's currently hard to get them from RUM, a recurrent theme is that knowing the resources processing time would be useful for understanding their visual impact.

Use cases I encountered so far:

• Image processing time would enable developers to estimate when these images were painted to screen. That can be used along with Element Timing, or as a polyfill for Element Timing. Because images are decoded in a progressive manner, we may need a list of events in that case.
• Font processing time would enable developers to estimate when the fonts were paints to screen.
• JS processing time would enable developers to take note that the performance impact of JS goes beyond its download. There may be a case in favor of splitting that between parsing (one time event) and execution (potentially ongoing). The "ongoing" case has some overlap with LongTasks, so that will require some thought.

Does it make sense to add that to Resource Timing? Or should we try to split it out into a separate spec that hooks into Resource Timing, in a similar way to Server Timing?

Start by reading the Resource Timing specification and compare the existing Element Timing and Server Timing approaches mentioned in the issue. Clarify whether resource processing events belong in Resource Timing or a separate specification, including image, font, and JavaScript cases. Done means a documented, agreed scope and integration direction.